A phase qubit can be a current-biased Josephson junction, operated in the zero voltage state with a non-zero current bias. A Josephson junction is a tunnel junction, made of two pieces of superconducting metal separated by a very thin insulating barrier, for example, about one nanometer in thickness. The insulating barrier is sufficiently thin to allow electrons, or in the superconducting state, Cooper-paired electrons, to tunnel through the barrier at an appreciable rate. Each of the superconductors that make up the Josephson junction can be described by a macroscopic wavefunction, as described by the Ginzburg-Landau theory for superconductors. The difference in the complex phases of the two superconducting wavefunctions is the most important dynamic variable for the Josephson junction, and is called the phase difference, or phase, of the Josephson junction.